The present invention relates to the field of the so-called xe2x80x9cintelligentxe2x80x9d array antennas, and more in particular to a discrimination procedure of a wanted signal from a plurality of cochannel interferents received by array antennas of base transceiver stations for cellular telecommunication and relative receiver.
1. Background Art
The use in mobile radio environment of antenna consisting of one or more arrays of electromagnetic field sensors (array) is known, for instance, from the European patent application published under No. 0593822A1. This document in fact, describes, mentioning the first claim, xe2x80x9cAn arrangement of antennas for a base transceiver station including a plurality of antenna arrays, each array able to form a multiplicity of narrow radiation lobes, separated and partially overlapped in the azimuth plane, the arrays being positioned in such a way that all the lobes generated by the same give a substantial omnidirectional coverage on said plane; lobes formation means (beamforming) in azimuth and angular elevation for each said array; a plurality of radiofrequency transceivers each one to transmit and receive the signals relative to one or more channels; a switching matrix to connect each transceiver with one or the other array through said beamforming means; etc.xe2x80x9d.
More in particular, a beam former, for example a Butler matrix, is associated to an antenna array. A beam former consists of phase shifting and adder circuits, used in reciprocal way both in reception and in transmission. Considering for instance reception, the beam former has a plurality of input ports for the signals coming from the sensors of the array , and a multiplicity of output ports, each one relative to a pre-set azimuth direction and corresponding to a particular combination of module and phase of input signals. A dual behaviour applies to transmission, where the same port selected during reception is used to transmit towards the mobile . The antenna array, and relative beam former, are therefore an essential part of a system capable to identify the direction of the signal transmitted uplink by a mobile, that the system follows condensing a narrow radiation lobe in which the power of the signal transmitted down-link towards the mobile itself is concentrated. This equals to an intelligent behaviour of the antenna, which deviates from the traditional utilization of antennas in the same sector of the technique. Thanks to the intelligent behaviour, the interference from cochannel channel is reduced and the re-utilization of the same frequencies in adjacent cells is made possible. The deriving advantage is considerable and consists in the possibility to increase the dimension of cells, at equal transmitted power, in low traffic areas or reduce the frequency re-utilization distance, increasing the number of carriers per cell in high traffic areas.
The estimate of wanted signal arrival direction avails of means for the measurement of signals present at output ports of the beam formers and of a processor evaluating the above mentioned measures and selecting the best direction, for instance that one for which the signal level is higher.
The antenna arrangement described in the mentioned application, overcomes the limits introduced by sectorial antennas in corner-excited cells, still widely employed in the mobile radio field, due to need of infrasectorial handovers as the position of the mobile part around common antennas varies, though remaining close to the same. The intelligent antenna system enables in fact a transceiver to be associated to any possible narrow radiation lobes, sunburst arranged on the azimuth plane, therefore the cell appears to the network as if it were equipped of omnidirectional antenna, but without the relative known drawbacks.
2. Background Art
Notwithstanding all the features mentioned above, the intelligent antenna arrangement described in the mentioned application, fails to exploit the capability of an antenna array to shape the overall radiation diagram so that nulls are steered in the interferents"" directions. For example, if interfering and wanted signal arrival directions are different but contained in the same beam, no discrimination can be made between them. This stands also if beam is selected, after demodulation, on the basis of wanted signal quality.
In order to exploit the interference nulling capabilities of the antenna array a numerical (and not analog) beamforming has to be performed on digitalized signals from the sensors of the array.
Numerical beamforming is nothing more then a linear combination of base band signals from the array elements using a set of complex coefficients w.
The procedures for numerical beamforming mainly differ in the choice of the coefficient""s set w and usually 2 different methods can be used (see [1] for a more detailed description and for a comparison based on system capacity):
1) All cochannel user arrival directions are estimated and coefficients w are to chosen with the constraint that the overall array gain is xe2x80x9cnulledxe2x80x9d in the interferent arrival directions. See also |2| for an overview of known art about beamforming and xe2x80x9cnull steeringxe2x80x9d.
2) Coefficients w are calculated so that the difference between the combined signal and a reference part of the signal is minimized.
Both these methods for coefficients w calculation suffer from the fact that they optimize uplink receiver performance but the coefficients w are not always reliable for downlink transmission.
For example, when uplink noise level is negligible and the interferent and wanted signal arrival directions are close each other, both methods try to reduce interference as much as possible. The result is often a very low level of the array gain in the direction of the wanted signal and thus only a small fraction of transmitted energy will be directed towards the user.
WO 95/22873 discloses a cellular system receiver including a spatial filter which has up to one input for each antenna element and one output for each spatial channel. An adaptor adjust the spatial filtering in order to enhance one of the channels while suppressing the other. A channel estimator calculates estimated impulse response for each selected channel based upon training sequences and the output from the spatial filter.
In particular adaptation means are disclosed suitable to calculate the weights w that implement the spatial filtering of signal coming from a sensor array. The above description however does not contain any detailed explanation about the proposed algorithm but only mention is made to known or general procedures like xe2x80x9cdirection of arrival estimationxe2x80x9d, xe2x80x9cminimisation of error signalxe2x80x9d and xe2x80x9cgradient optimisation methodxe2x80x9d. Neither the cost function to be minimised nor the method to find that minimum is provided in a detailed text.
EP 809323 discloses a signal processing apparatus e.g. for wireless communication system generating beam pattern which has its maximum gain along direction of wanted signal and maintaining gain toward direction of interfering signals in as low level as possible. In particular a method to calculate said weights w is disclosed and according to this method the weights are chosen so to maximizse received wanted signal power. Interference is reduced as a consequence of directivity in the resulting array diagram, but is not minimised placing nulls in the interfering signal arriving directions.
The scope of the present invention is to overcome the above mentioned drawbacks and to indicate a discrimination procedure of a wanted signal from a plurality of cochannel interferents received by array antennas of base transceiver stations for cellular telecommunication.
To attain these objects, scope of the present invention is a discrimination procedure of a wanted signal from a plurality of cochannel interferents received by array antennas of frequency division (FDMA), or time division (TDMA), or mixed FDMA/TDMA multiple access telecommunication systems, re-employing a same frequency group in adjacent territorial areas, including an estimate phase of the arrival directions of the said interferents and of said wanted signal, and a successive phase of spatial filtering in which signals transduced by the relative sensors of a said array are linearly combined among them through multiplication coefficients, or weights, obtaining a reception signal cleaned from interferents; said phases being repeated for each one of the time slots, in the TDMA frame, in which the wanted user transmits, characterized in that said weights satisfy the two following conditions:
A) the gain of said array in the ratio between said wanted signal and the noise after spatial filtering, compared to the traditional use of a single sensor, is constrained so that it is is not less than a properly selected threshold;
B) the ratio between the sum of interferents"" powers and wanted signal power is minimized, as described in claim 1.
According to the above, in the algorithm disclosed in the present application weights w are chosen so minimise intereference, placing nulls in the interfering signal arrival directions, while keeping a minimum value for the array gain in the wanted signal direction.
In EP 0 809 323, according to which the preamble of the main claim is drafted, weights w are chosen so to maximise, as above explained received wanted signal power. According to the above, the teaching of the present application are therefore contrary to the teaching of said EP patent.
The subject procedure can find useful application in the realization of a receiver for base transceiver stations of cellular telephone systems of the GSM 900 MHz, or DCS 1800 MHz type.
The occurrence of condition A) enables to obtain some advantages:
1. To improve the performance in uplink reducing the performance sensitivity of the spatial filtering to the error made in the estimate of wanted signal arrival directions.
2. To enable, in downlink, a high directivity of the antenna array radiation diagram in the direction of the wanted.
The occurrence of condition B) enables to improve the performance in the two connections, uplink and downlink, reducing the interference.
Profitably, it is possible to express in the vectorial form the equations concerning the estimate phases of the arrival direction and spatial filtering, as well as those generated by the imposition of the above mentioned featuring conditions, thus making the mathematical computation for the determination of said weights more compact.
Further object of the present invention is a frequency division multiple access receiver for telecommunication systems (FDMA), or time division multiple access (TDMA), or mixed (FDMA/TDMA), which re-employ a same frequency group in adjacent territorial areas, characterized in that it includes means for the actuation of the discrimination procedure of a wanted signal from a plurality of cochannel interferents, which already formed the object of an invention, as described in claim 13.